In recent years, efforts have been made for making practical use of elements which have a micro-structure formed by micromachining technology. For example, in the field of optical communications technology, micromirror elements, which are very small elements capable of reflecting light, are gathering attention.
In the optical communications, optical fibers serve as a medium through which optical signals are passed. When the optical signal passing through a given optical fiber is switched to another optical fiber, so-called optical switching devices are used in general. In order to achieve high quality optical communications, the optical switching device must have such characteristics as high capacity, high speed and high reliability, in switching action. In view of these, micromirror elements manufactured by utilizing micromachining technology are gathering attention as a switching element to be incorporated in the optical switching devices. The micromirror elements enable the switching operation without converting optical signals into electric signals between the optical paths on the input side and the output side of the optical switching device. This is advantageous in achieving the above-mentioned characteristics.
Micromirror elements have mirror surfaces for reflecting light, and are capable of varying the direction of light reflection by pivoting the mirror surfaces. Many micromirror elements which are used in optical devices are electrostatically-driven. The electrostatically-driven micromirror elements are roughly divided into two types: the micromirror element manufactured by so called surface micromachining technology, and the micromirror element manufactured by so called bulk micromachining technology.
In the surface micromachining technology, material thin film is formed on a substrate, into a desired pattern according to component parts to be made. Such a pattern is laminated in sequence to form supports, mirror surfaces, electrodes and other component parts which constitute the element as well as sacrifice layers which will be removed later.
On the other hand, in the bulk micromachining technology, etching is made to a material substrate itself, whereby supports, mirror regions and so on are made into desired shapes. Thin-film formation is performed as necessary, to form mirror surfaces and electrodes. Technical details of the bulk micromachining technology are disclosed in JP-A 10-190007, JP-A 10-270714 and JP-A 2000-31502, for example.
Micromirror elements must satisfy a technical requirement that the mirror surface which is responsible for reflecting light should be highly flat. However, according to the surface micromachining technology, the finalized mirror surface is thin and susceptible to warp. For this reason, a high flatness is guaranteed only within an area of mirror surface measured by a side length of up to 50 μm.
On the contrary, according to the bulk micromachining technology, a relatively thick material substrate is etched to form mirror regions, and mirror surfaces are made on this mirror regions. Therefore, it is possible to provide rigidity to a larger area of mirror surface, and as a result, it is possible to form mirror surfaces which have sufficiently high optical flatness. For this reason, bulk micromachining technology is employed widely particularly in the manufacture of micromirror elements whose mirror surface must have a side length not smaller than 100 μm.
FIG. 32 is a partially non-illustrated exploded perspective view of a micromirror element 400 as an example of a conventional, electrostatically-driven micromirror element manufactured by the bulk micromachining technology. The micromirror element 400 has a laminated structure in which a mirror substrate 410 and a base substrate 420 are laminated via a spacer (not illustrated). The mirror substrate 410 includes a mirror region 411, an inner frame 412 and an outer frame 413. The mirror region 411 and the inner frame 412 are connected with each other by a pair of torsion bars 414. The inner frame 412 and the outer frame 413 are connected with each other by a pair of torsion bars 415. The pair of torsion bars 414 provides a center axis for a pivotal action of the mirror region 411 with respect to the inner frame 412. The torsion bars 415 provide a center axis for a pivotal action of the inner frame 412 with respect to the outer frame 413 and of the mirror region 411 associating therewith.
The mirror region 411 has a back surface provided with a pair of flat-plate electrodes 411a, 411b, and a top surface provided with a mirror surface (not illustrated) for refection of light. The inner frame 412 has a back surface provided with a pair of flat-plate electrodes 412a, 412b. 
The base substrate 420 is provided with: flat-plate electrodes 420a, 420b which are faced to the flat-plate electrodes 411a, 411b of the mirror region 411; and flat-plate electrodes 420c, 420d which are faced to the flat-plate electrodes 412a, 412b of the inner frame 412. Such flat-plate electrodes are often used in conventional micromirror element as driving means in order to generate electrostatic force.
According to the above construction, when the flat-plate electrode 411a of the mirror region 411 is positively charged for example, and the flat-plate electrode 420a of the base substrate 420 is negatively charged, then electrostatic attraction is generated between the flat-plate electrodes 411a and the flat-plate electrodes 420a, which pivots the mirror region 411 in a direction indicated by Arrow M1 while twisting the torsion bars 414.
On the other hand, for example, when the flat-plate electrode 412a of the mirror region 412 is positively charged, and the flat-plate electrode 420c of the base substrate 420 is negatively charged, then electrostatic attraction is generated between the flat-plate electrodes 412a and the flat-plate electrode 420c, which pivots the mirror region 412 together with the mirror region 411 in a direction indicated by Arrow M2 while twisting the torsion bars 415. FIG. 33 shows a situation in which the inner frame 412 and the associating mirror region 411 are displaced at an angle θ with respect to the outer frame 413.
According to the flat-plate electrode structure which is utilized in the micromirror element 400, the flat-plate electrodes 420a, 420b, 420c, 420d in the base substrate 420 tend to draw the mirror region 411 which is provided with the flat-plate electrodes 411a, 411b or the inner frame 412 which is provided with the flat-plate electrodes 412a, 412b for eventual contact. Therefore, there is a pull-in voltage between a pair of electrodes; specifically, when a voltage applied between a pair of electrodes exceeds the pull-in voltage, the mirror region 411 and/or the inner frame 412 can begin to approach the base substrate 420 at a wildly accelerated speed in the course of pivoting action, resulting in a problem that the tilting angle of the mirror region 411 cannot be controlled appropriately. The problem is particularly egregious when a large tilting angle (5° approx. or greater) must be achieved, i.e. when the torsion bars are twisted to a large extent.
In an attempt to solve this problem, proposals are made in which the micromirror element is driven by a comb-teeth electrode structure instead of the flat-plate electrode structure. FIG. 34 is a partially non-illustrated perspective view of a micromirror element 500 which utilizes a comb-teeth electrode structure.
The micromirror element 500 includes a mirror region 510 which has an upper surface or a lower surface provided with a mirror surface (not illustrated), a mirror region 510, inner frame 520 and an outer frame 530 (partially non-illustrated). Each component is integrally formed with a comb-teeth electrode. Specifically, the mirror region 510 has a pair of mutually parallel sides formed with a pair of outwardly extended comb-teeth electrodes 510a, 510b. The inner frame 520 is formed with a pair of inwardly extended comb-teeth electrodes 520a, 520b correspondingly to the comb-teeth electrode 510a, 510b, and with a pair of outwardly extended comb-teeth electrodes 520c, 520d. The outer frame 530 is formed with a pair of inwardly extended comb-teeth electrodes 530a, 530b correspondingly to the comb-teeth electrode 520c, 520d. The mirror region 510 and the inner frame 520 are connected with each other by a pair of torsion bars 540. The inner frame 520 and the outer frame 530 are connected with each other by a pair of torsion bars 550. The torsion bars 540 provide a center of axis for the pivotal action of the mirror region 510 with respect to the inner frame 520. The torsion bars 550 provides a center of axis for the pivotal action of the inner frame 520 with respect to the outer frame 530 and a center of axis for the associating pivotal action of the mirror region 510.
According to the micromirror element 500 which has the structure as described, a set of comb-teeth electrodes which are placed closely to each other for generation of electrostatic force, e.g. the comb-teeth electrode 510a and the comb-teeth electrode 520a, are apart from each other, making an upper and a lower steps as shown in FIG. 35A when no voltage is applied. When a voltage is applied, as shown in FIG. 35B, the comb-teeth electrode 520a pulls the comb-teeth electrode 510a in, thereby driving the mirror region 510. More specifically, in FIG. 34, when the comb-teeth electrode 510a is positively charged and the comb-teeth electrode 520a is negatively charged, then the mirror region 510 pivots in a direction indicated by Arrow M3 while twisting the torsion bars 540. On the other hand, when the comb-teeth electrode 520c is positively charged and the comb-teeth electrode 530a is negatively charged, then the inner frame 520 pivots in a direction indicated by Arrow M4 while twisting the torsion bars 550.
According to the comb-teeth electrode, the electrostatic force developed between the electrodes acts in directions which are generally perpendicular to the pivoting directions of the mirror region 510. Therefore, when the mirror region 510 is driven, the distance between the electrodes in directions in which the electrostatic force acts is generally constant, and thus, the comb-teeth electrodes are unlikely to contact with each other due to the pull-in. This enables to attain a large tilting angle of the mirror region 510 appropriately.
According to the micromirror element 500, pivoting action of the mirror region 510 and the inner frame 520 causes a displacement of the comb teeth (electrode), and therefore it is necessary that the comb-teeth electrodes are formed to have a sufficient thickness for predetermined tilting angle of the mirror region 510 and the inner frame 520. For example, if the mirror region 510 has a body region 511 which has a length D of 1 mm, and the mirror region 510 is pivoted with respect to the inner frame 520, by 5 degrees around an axis provided by a pair of torsion bars 540, one of two body end regions 511′ dips down by 44 μm. For this reason, the mirror region 510 must be formed with comb-teeth electrodes 510a, 510b which have a thickness T of at least 44 μm.
On the other hand, from the view point that a large tilting angle should be attained with a small voltage, it is desirable that the torsion bars 540, 550 which have torsional resistance should be thin. However, in the conventional micromirror element 500, the torsion bars 540, 550 are formed to have the same large thickness with the material substrate which constitutes the mirror region 510, the inner frame 520 and the outer frame 530. For example, if the comb-teeth electrodes 510a, 510b are designed to have a thickness T of 44 μm or greater as described above, both the mirror region 510 and the torsion bars 540, 550 will be 44 μm or greater. If the torsion bars 540, 550 are as thick as this, a large electrostatic force must be generated between the comb-teeth electrodes in order to twist these torsion bars, and as a result, a large driving voltage must be supplied. Although torsional resistance of the torsion bars 540, 550 are adjusted by varying the width of torsion bars 540, 550 according to the conventional art, design change in the width is often not enough to attain an appropriate torsional resistance.
As described, according to the microstructures manufactured by the bulk micromachining technology, it is sometimes necessary that structural regions which are formed by etching the material substrate have different thicknesses or heights. However, in the conventional bulk micromachining technology, it is difficult to form a thin structural region or a thin wall portion which has a highly accurate thickness dimension, integrally with a thick structural region or a thick wall region.